1. Field of the Invention
The present invention relates to a mold for forming glass, particularly to a mold for forming glass, which is useful for press-molding a glass product such as a panel or funnel for a cathode ray tube (CRT) for TV.
2. Discussion of Background
A CRT for TV is produced in such a manner that a front panel, a funnel and a neck are prepared separately, and after applying phosphors to the inner surface of the panel and attaching a shallow mask and electrodes, they are bonded to form a CRT. Therefore, very strict quality control is required for the properties such as the surface irregularities, the surface roughness of the inner surface of the panel.
FIG. 1 shows a cross sectional schematic view illustrating the molding of a panel glass for a CRT. Molding of the panel is carried out by pressing a molten glass at a high temperature of about 1,000.degree. C. by a mold. Therefore, the mold for forming glass is required to have mechanical strength and heat resistance and further required to have a property such as chemical stability against a high temperature glass.
As a mold for forming glass, which is suitable for such a purpose, one having chromium plating or nickel tungsten alloy plating applied on stainless steel, has been commonly used. The plated coating film undergoes deterioration by use, and the mold body is reused by removing the coating film and applying plating afresh. To carry out chromium plating which is suitable for the mold, a plating bath such as a sergent bath, a mixed catalyst bath or a high efficiency bath, is used, and each contains highly toxic hexavalent chromium.
Further, as shown in FIG. 1, when molten glass charged in a bottom mold 13, is pressed by a plunger 11 for molding and the plunger 11 is withdrawn from the solidified panel 10, burr-like brittle marks (FIG. 2) are likely to form on the sidewall portion 14 of the plunger due to friction with the inside wall of the panel. In every molding operation, glass enters into such brittle marks formed on the sidewall portion of the plunger 11, and when the plunger 11 ascends, the brittle marks will abrade the entered glass to bring about scuff defects on the inside surface of the panel 10, and checks are likely to start from such defects, thus leading to deterioration of the quality of the panel.
To prevent this, it is necessary to apply a releasing agent frequently. Further, a plated coating film of chromium has a drawback such that it is extremely weak against halogen-type impurities.
On the other hand, a plated coating film of nickel or cobalt, or an alloy containing such an element as the main component is excellent in a releasing property and free from formation of brittle marks which is likely to take place with chromium plating, and it tends to have ductile marks in to which glass hardly enters. Thus, it has a feature that marks which bring about kinky threads on the inner surface of the panel, hardly form. For example, nickel-tungsten alloy plating can be used for film forming under such conditions that there will be no problem of toxicity or odor of reagents, whereby no scuff tends to form on the glass surface, and the frequency of using a releasing agent can be reduced. Accordingly, its use has recently been increased. However, a plated coating film of nickel or cobalt, or an alloy containing such an element as the main component, is likely to undergo, under a high temperature condition, mutual diffusion with iron in the stainless steel as the matrix material of the plunger and is readily oxidized, so that a chemically and mechanically stable oxide film tends to be hardly formed at the coating film surface. Accordingly, the surface roughness and the surface condition of the inner surface of the face panel presenting an image, which are most important from the viewpoint of the panel quality, tend to deteriorate, and it will be required to change the plunger after from 10,000 to 15,000 times of pressing operation, to remove the plated coating film and to carry out plating afresh. Such a coating film has had a drawback that the useful life of the coating film is very short as compared with the above mentioned plated coating film of chromium which is likely to bring about formation of scuff defects on the inner surface of the panel.